Increasingly tight environmental regulations for NO.sub.x emission for coal-, oil- and gas-fired utility boilers are urging utility and industrial users of fossil fuels to pay greater attention to control of NO.sub.x in oil, coal and even gas-fired units. Effective control of NO.sub.x emissions requires the application of one or a combination of methods of combustion process modification including staged air and staged fuel injection, the use of low-NO.sub.x burners, and post-combustion clean-up such as NH.sub.3 injection into combustion gases.
The most widely used design strategy for NO.sub.x reduction is staged combustion. Fuel-rich and -lean combustion zones in flames are created by "staging" the input of either air (overfire air) or fuel with injections positioned at selected axial points along the combustion stream.
In the case of "internal staging" processes, fuel-rich and -lean combustion zones are produced by appropriate mixing of the fuel and air introduced from a single burner stage, rather than by producing fuel-rich and -lean combustion zones using different axial stages (physically separated stages). Systems of this type employing multi-annular burner stages are used for example by Beer U.S. Pat. No. 4,845,940 and Beer et. al., U.S. Pat. No. 4,539,818.
The degree of NO.sub.x reduction achieved by these technologies has been observed to vary widely and to depend on the combustion system.